Consolidated Balance Sheet (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Current Assets:
Cash and cash equivalents	$ 298	$ 255
Accounts receivable - trade (less allowance for doubtful accounts)	523	473
Unbilled revenue	265	324
Miscellaneous accounts and notes receivable	82	69
Materials and supplies	756	712
Mark-to-market derivative assets	134	115
Current regulatory assets	250	215
Other current assets	98	132
Total current assets	2,406	2,295
Property and Plant, Net	17,833	18,127
Investments and Other Assets:
Nuclear decommissioning trust fund	407	357
Goodwill	411	411
Intangible assets	14	7
Regulatory assets	1,655	1,603
Other assets	772	845
Total investments and other assets	3,259	3,223
TOTAL ASSETS	23,498	23,645
Current Liabilities:
Current maturities of long-term debt	206	179
Short-term debt	5	148
Accounts and wages payable	458	693
Taxes accrued	163	65
Interest accrued	145	101
Customer deposits	96	98
Mark-to-market derivative liabilities	155	161
Current regulatory liabilities	125	133
Other current liabilities	193	207
Total current liabilities	1,546	1,785
Long-term Debt, Net	6,781	6,677
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,385	3,315
Accumulated deferred investment tax credits	74	79
Regulatory liabilities	1,542	1,502
Asset retirement obligations	429	428
Pension and other postretirement benefits	1,152	1,344
Other deferred credits and liabilities	563	447
Total deferred credits and other liabilities	7,145	7,115
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,611	5,598
Retained earnings	2,259	2,369
Accumulated other comprehensive gain (loss)	2	(50)
Stockholder's equity	7,874	7,919
Noncontrolling Interest	152	149
Total equity	8,026	8,068
TOTAL LIABILITIES AND EQUITY	23,498	23,645
Union Electric Company [Member]
Current Assets:
Cash and cash equivalents	132	201
Accounts receivable - trade (less allowance for doubtful accounts)	263	212
Accounts receivable - affiliates	3	1
Unbilled revenue	127	139
Miscellaneous accounts and notes receivable	54	42
Materials and supplies	402	348
Current regulatory assets	147	109
Other current assets	63	82
Total current assets	1,191	1,134
Property and Plant, Net	10,092	9,958
Investments and Other Assets:
Nuclear decommissioning trust fund	407	357
Intangible assets	12	7
Regulatory assets	813	855
Other assets	452	446
Total investments and other assets	1,684	1,665
TOTAL ASSETS	12,967	12,757
Current Liabilities:
Current maturities of long-term debt	205	178
Accounts and wages payable	187	414
Accounts payable - affiliates	54	73
Taxes accrued	133	74
Interest accrued	68	62
Current regulatory liabilities	34	57
Other current liabilities	114	84
Total current liabilities	795	942
Long-term Debt, Net	3,806	3,772
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,322	2,132
Accumulated deferred investment tax credits	66	70
Regulatory liabilities	906	836
Asset retirement obligations	342	328
Pension and other postretirement benefits	432	491
Other deferred credits and liabilities	161	149
Total deferred credits and other liabilities	4,229	4,006
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	1,555	1,555
Preferred stock not subject to mandatory redemption	80	80
Retained earnings	1,991	1,891
Stockholder's equity	4,137	4,037
TOTAL LIABILITIES AND EQUITY	12,967	12,757
Ameren Illinois Company [Member]
Current Assets:
Cash and cash equivalents	93	21
Accounts receivable - trade (less allowance for doubtful accounts)	197	201
Accounts receivable - affiliates	9	15
Unbilled revenue	108	146
Miscellaneous accounts and notes receivable	11	6
Materials and supplies	202	199
Current regulatory assets	161	306
Current accumulated deferred income taxes, net	46	58
Other current assets	47	65
Total current assets	874	1,017
Property and Plant, Net	4,948	4,770
Investments and Other Assets:
Tax receivable - Genco	40	56
Goodwill	411	411
Regulatory assets	842	748
Other assets	112	211
Total investments and other assets	1,405	1,426
TOTAL ASSETS	7,227	7,213
Current Liabilities:
Current maturities of long-term debt	1	1
Accounts and wages payable	148	133
Accounts payable - affiliates	82	103
Taxes accrued	12	15
Interest accrued	37	22
Customer deposits	74	76
Mark-to-market derivative liabilities	78	99
Mark-to-market derivative liabilities - affiliates	59	200
Environmental remediation	38	63
Current regulatory liabilities	91	76
Other current liabilities	62	70
Total current liabilities	682	858
Long-term Debt, Net	1,727	1,657
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,009	895
Accumulated deferred investment tax credits	6	7
Regulatory liabilities	636	666
Pension and other postretirement benefits	444	495
Other deferred credits and liabilities	276	183
Total deferred credits and other liabilities	2,371	2,246
Commitments and Contingencies
Stockholders' Equity:
Common Stock
Other paid-in capital	1,965	1,965
Preferred stock not subject to mandatory redemption	62	62
Retained earnings	406	408
Accumulated other comprehensive gain (loss)	14	17
Stockholder's equity	2,447	2,452
TOTAL LIABILITIES AND EQUITY	7,227	7,213
Ameren Energy Generating Company [Member]
Current Assets:
Cash and cash equivalents	25	8
Advances to money pool	32	74
Accounts receivable - affiliates	62	89
Miscellaneous accounts and notes receivable	15	13
Materials and supplies	117	122
Other current assets	23	19
Total current assets	274	325
Property and Plant, Net	2,288	2,231
Investments and Other Assets:
Other assets	13	16
TOTAL ASSETS	2,575	2,572
Current Liabilities:
Accounts and wages payable	62	71
Accounts payable - affiliates	11	13
Current portion of tax payable - Ameren Illinois	6	8
Taxes accrued	13	20
Interest accrued	27	13
Mark-to-market derivative liabilities	9	3
Other current liabilities	10	14
Total current liabilities	138	142
Long-term Debt, Net	824	824
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	350	304
Accumulated deferred investment tax credits	2	2
Tax payable - Ameren Illinois	40	56
Asset retirement obligations	59	66
Pension and other postretirement benefits	66	141
Other deferred credits and liabilities	18	12
Total deferred credits and other liabilities	535	581
Commitments and Contingencies
Stockholders' Equity:
Common Stock
Other paid-in capital	656	653
Retained earnings	445	437
Accumulated other comprehensive gain (loss)	(33)	(72)
Stockholder's equity	1,068	1,018
Noncontrolling Interest	10	7
Total equity	1,078	1,025
TOTAL LIABILITIES AND EQUITY	$ 2,575	$ 2,572